Liquid epoxy-derived acrylate resins are generally prepared by the reaction of an acrylic type acid and a polyepoxide. The practical synthesis of these acrylate resins requires the use of a nucleophilic catalyst such as tetramethylammonium chloride (TMAC), triphenylphosphine, tetramethylammonium bromide (TMAB), tetramethylammonium iodide (TMAI), triphenylethylphosphonium iodide or triethanolamine. For example when triphenylphosphine is used, the reaction may be depicted as follows: ##EQU1##
The liquid acrylate type resins are very attractive to end users because of their inherent low viscosity and high reactivity. But this inherent high reactivity is a source of problems in that it is difficult for the manufacturer to repeatedly produce resins having the same initial viscosity. Any slight variation in process conditions can cause a variation in the resin's initial viscosity. Furthermore, this high reactivity presents a problem if the resins must be stored for long periods of time or shipped during hot weather. The end user wants a liquid resin with a repeatable low viscosity and high reactivity yet good storage stability.
A known source of instability in the acrylate type resins is the tendency to undergo free radical polymerization. This, of course, is the very mechanism used by the end user to cure the resins. It is known to add free radical inhibitors to acrylate resins and in fact, free radical inhibitors do enhance the stability of the acrylate type resins of this invention. However, an 85% acrylate resin (where the % acrylate resin means the approximate % of epoxide groups reacted with acrylic type acid) made using a nucleophilic catalyst without "added" free radical inhibitor will double its viscosity in 2 days at 60.degree.C but the same acrylate resin with additional free radical inhibitor will also double its viscosity in 2 days at 60.degree.C. By "added" free radical inhibitor is meant that inhibitor over and above that normally used in acrylate resins. Almost all acrylates including the acrylic type acids used to make the ester composition of this invention have hydroquinone in them to retard premature polymerization. As can be seen, even with the free radical inhibitor the acrylate resins made with a nucleophilic catalyst exhibit a high degree of instability. The end user wants to use a low viscosity resin with little variation from lot-to-lot in its viscosity, a resin which can be stored under a variety of conditions for different lengths of time, and a resin which has high reactivity. It has now been found that the lot-to-lot viscosity variability and the stability of the acrylate resins can be dramatically enhanced, thereby solving the problems faced by the manufacturer and end user, by the addition of selected amounts of certain acids.